"Microlithography" encompasses several general methods, including "photolithography" (lithography employing light as an exposure medium) and "charged-particle-beam projection lithography" (lithography employing, e.g., an electron beam).
In photolithography, a light source directs light toward a mask to expose the mask pattern onto a sensitive substrate, such as a silicon wafer. The mask typically comprises a pattern of features defined in a chromium layer applied to a glass substrate. During use of the mask, light is illuminated onto the mask; regions of the mask in which no chromium is present transmit the incident light, and regions of the mask in which chromium is present reflect and/or absorb the incident light.
The pattern exposed on the wafer should ideally match the desired pattern as defined by the mask. For example, light passing through a mask feature having a rectangular profile should ideally form a corresponding feature having a rectangular profile on the wafer. However, when edges of adjacent mask features are in close proximity to each other, light passing through the mask in the vicinity of the edges is diffracted. The result of such diffraction and related effects is that the corresponding features formed on the wafer do not have the same profile of the corresponding features on the mask. For instance, the line width of an affected feature on the wafer may be narrower than expected based on the line width of the corresponding feature on the mask, and/or a corner of an affected feature on the wafer may be radiused while the corresponding feature on the mask has a sharp corner. This effect, known as the "optical proximity effect," can be especially problematic in high-density semiconductor devices.
There have been several methods proposed for reducing the optical proximity effect. For example, Haruki et. al., "Mask Pattern Correction Tool Using Genetic Algorithm," Digest of Papers Microprocess '96 (9.sup.th International Microprocess Conference, 1996), p. 102, 1996, discloses a method in which the profiles of the features to be transcribed are altered. In the Haruki et al. method, the profile of individual features is altered by square and/or rectangular irregularities, such that the edges of individual features have "serifs". However, since the optical proximity effect often produces radiused corners in features that nominally should have sharp corners, the corrective precision of the Haruki et al. method is insufficient. Although designing the serif profiles is normally done using a computer, the calculations are time-consuming. Furthermore, the number of serifs that have to be transcribed is so high that pattern-delineation throughput tends to be very low.
In electron-beam or other charged-particle-beam projection lithography, proximity effects are also a serious problem. Such effects are caused by diffraction of the electron beam (or other charged-particle beam) around the edges of adjacent features on the mask. The general term "proximity effect" as used herein encompasses the optical proximity effect and proximity effects arising during use of a charged-particle beam.